An electro-photography (EP) printing device forms an image on media typically by first selectively charging a photoconductive drum in correspondence with the image. Colorant is applied to the photoconductive drum where the drum has not been charged, and then this colorant is transferred to the media to form the image on the media. Traditionally, the most common type of EP printing device has been the laser printer, which is a dry EP (DEP) printing device that employs toner as the colorant in question. More recently, liquid EP (LEP) printing devices have become popular.
An LEP printing device employs ink, instead of toner, as the colorant that is applied to the photoconductive drum where the drum has been charged. The ink includes solid pigment particles within a carrier liquid. To ensure proper LEP printing, the concentration of the solid pigment particles within the carrier liquid is desirably maintained at a substantially constant level for a given type of ink. Thus, the concentration of the colorants within the carrier liquid is desirably measured.